Nope, the Mortal Realms coalesced into being from the remnants of the World That Was. As you said you didn't follow AoS lore, this little video from GW is a handy and quick guide to the Mortal Realms:

As to the question of power level, it's important to note that gods in AoS aren't impossibly powerful. Sigmar was unable to withstand the forces of Chaos and had to retreat to Azyr after losing Ghal Maraz. Slaanesh, a Chaos god and unquestionably one of the most powerful beings in existence, was imprisoned by Teclis and Tyrion. Shyish is invaded by Chaos during the Age of Chaos and Nagash is actually slain by Archaon at the Battle of the Black Skies. His mortarchs recover his body and transport it to Stygxx where he regenerates over the course of centuries before he returns and drives Chaos out of Shyish. So the fact that Nagash is a god isn't a de facto indication of power level. Archaon is not a god, and he slew Nagash.

Furthermore, when Nagash sees the culmination of his grand plan during the Time of Tribulation, he's not doing it all based around his own power. Nagash has been transporting the realmstone of Shyish from the edge of the Realm, where it's typically found in the greatest concentration, and bringing it to the centre of the Realm in order to invert the flow of magic across all of Shyish. His own magics are what raises up the great pyramids that eventually form the nexus of inversion, but it's quite clear that this is no simple divine act where he can simply snap his fingers and everything falls into place. It's a vast effort brought about primarily by the endless labour of countless numbers of undead workers.

Now, in contrast to this, we have Kroak. Little has been written in AoS about the last remaining First Generation Slann, which is a pity since the Seraphon have some very interesting fluff in AoS. But there's some intriguing snippets as to his power in our Battletome:

Time and their long journey across the void have taken their toll upon the slann. Many have been slain in the relentless war with the Dark Gods, while others have simply vanished, lost to the slow march of ages and the endless battles that wrack the Mortal Realms. However, some live on despite the death of their bodies as spirits trapped within desiccated mortal forms – creatures of celestial magic more than flesh and blood. Greatest among these is Lord Kroak, believed by the slann to be the oldest of their kind. Filled with the light of Azyr, Kroak is a vessel of immense arcane power, and though his body may be withered and useless, battlefields still tremble before his mighty incantations.

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Few creatures have mastered the secrets of magic like the slann. Long aeons spent traversing the stars have saturated their minds with the power of Azyr.

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So from this we know that Kroak is the most powerful of those Slann that have died yet clung onto existence. The second quote however is very interesting. What it implies is that the Slann have increased in power since fleeing the World That Was, thanks to being saturated in the power of Azyr during their long journey across the cosmos. Those Slann that have died but remained in existence have become creatures of celestial magic, and it would be a fair assumption that they have a greater level of saturation than a mortal Slann; indeed, Kroak is described as being "filled with the light of Azyr". With this in mind, Kroak's powers have almost certainly increased since leaving the World That Was, and given he was one of the most powerful magic users ever to exist on that world, his powers now would have to be truly immense.

That being said, the issue we have is that we simply have no bar to place Kroak against because of the paucity of fluff featuring him. One snippet in a Battletome that describes him as being awesome is hardly enough to form a definitive judgement of how strong he is; Battletomes, Army Books and Codices always present the characters within them as immensely powerful.

Hmmm yeah the thing with the gods in Warhammer is that they seem to be more like Norse gods or Greek gods, as in they can occasionally be defeated and especially outsmarted by (powerful or very smart) mortals, in contrast to the gods in christianity and other religions that are literally all-knowing and alllmighty so they always win except against each other (if there is such a thing as more than one god in that pantheon).

The Old Ones for one. They were clearly more powerful than Kroak, even though they disappeared (as far as I know their fate is not known).

The Chaos gods are infinitely more influential. You can field Kroak in both AoS and WFB, but you can't field a Chaos god because they would be completely OP and game breaking. In addition, Kroak on the battlefield (WFB) is less effective than Kairos, and Kairos is a mere servant to his master: Tzeentch. Also consider that the influence of Chaos (which is a composite of the 4 chaos gods) is far wider reaching than Kroak in terms of army books, followers, influence on geographical areas and general story progression.

In terms of AoS... you'd have to go with Sigmar. I mean the game is named after him! Plus his wretched SCE make up the heart of the entire game. Seraphon are more of a footnote (no model releases thus far and only an outdated battletome to speak of). Didn't Sigmar literally create the mortal realms? Kroak has done nothing so substantial.

There are more, but you get my point.

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Given that the old ones don't seem to actually be around anymore I wouldn't count em.

As for the chaos gods, the reason I'd currently give Kroak an edge is that Kroak survived the end times basicly unharmed, the chaos gods didn't. It took a while before the chaos gods were up and about again. Similarly Sigmar seemed to be quite shaken from the end times while Kroak and his fellow slanns merely continued doing whatever it is they do nowadays in their space ships. The main issue is that beyond being the only real survivors of the world that was we haven't really done anything, and the few fluff pieces of a slann or seraphon actually doing something are unimpressive to say the least... Right now the most influence we have had are some throw away sentences along the lines of "And then a slann showed up with his army and murdered a bunch of daemons" or "And then the temple guard stood guard for a millenia and when the slann finally awoke everything was littered with the corpses of those trying to test their might against the mythical guards". And even those bits of fluff lack detail as they never state how vital the thing the seraphon did there was.

So from this we know that Kroak is the most powerful of those Slann that have died yet clung onto existence. The second quote however is very interesting. What it implies is that the Slann have increased in power since fleeing the World That Was, thanks to being saturated in the power of Azyr during their long journey across the cosmos. Those Slann that have died but remained in existence have become creatures of celestial magic, and it would be a fair assumption that they have a greater level of saturation than a mortal Slann; indeed, Kroak is described as being "filled with the light of Azyr"

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Kroak's strength has definitely gone through major fluctuations across time. He was immensely strong while alive in the old world. After his death his spirit had only a fraction of the magical ability that he had in life (as per the 8th edition AB). And now, as you presented, he has become stronger during his voyage across the stars. It's difficult to say exactly how powerful he is now. Our analysis is further hindered by the fact that the Seraphon have a lesser role in AoS fluff.